What Is an Indicator?

Introduction

Since we cannot taste or touch chemicals to test them, we need something that will "tell" us whether a solution is acidic, alkaline or neutral. That something is an indicator . An indicator is a substance with a colour that depends on its surroundings. Add it to an acid and it shows one colour; add it to an alkali and it shows another. Indicators turn an invisible property into something you can see.

Core explanation

Definition. An indicator is a substance that changes colour when it is added to an acid or an alkali. The colour tells you which kind of solution you have.

How indicators are used. A few drops of indicator solution are added to a sample, or a strip of indicator paper is touched with a drop of the sample. You compare the colour with the known colours for acids and alkalis. Only a small amount of indicator is needed, and it does not noticeably change the solution being tested.

Three families of indicators.

Type Examples Acid colour Alkali colour --- --- --- --- Natural (plant or lichen) Litmus Red Blue Natural (plant) Turmeric Yellow Reddish-brown Natural (plant) Red cabbage extract Red or pink Green to yellow Synthetic Phenolphthalein Colourless Pink Synthetic Methyl orange Red Yellow Mixed Universal indicator Red, orange, yellow Blue, purple

Natural indicators come from living things. Litmus comes from lichens; turmeric is a spice; many coloured flowers, fruits and vegetables contain pigments that act as indicators.

Synthetic indicators are made by chemists. They give sharp, reliable colour changes, which makes them useful in accurate laboratory work.

Universal indicator is a mixture of several indicators. Because each one changes colour at a different level of acidity, the mixture shows a whole range of colours and can tell you roughly how acidic or alkaline a solution is, not just which group it belongs to.

Neutral solutions. Indicators also identify neutral solutions: litmus stays its original colour, and universal indicator turns green.

What makes a good indicator? A useful indicator shows clearly different colours in acid and alkali, gives the same result every time, and can be seen easily. Pale or muddy colours are hard to read, especially in coloured samples such as fruit juice.

Step-by-step reasoning

To use an indicator to classify a solution:

1. Choose an indicator and recall its acid and alkali colours. 2. Add a few drops of it to a small sample, or spot the sample onto indicator paper. 3. Observe the colour against a white background. 4. Match the colour to the known list. 5. Conclude whether the solution is acidic, alkaline or neutral.

Visual explanation

Imagine three test tubes in a rack, each with a few drops of red cabbage extract. The tube of vinegar is pink-red, the tube of pure water stays purple, and the tube of soap solution turns green. One indicator, three colours, three answers.

Real-world analogy

An indicator works like a mood ring that changes colour with temperature. You cannot feel the tiny temperature change, but the ring shows it. Likewise you cannot see acidity, but the indicator reveals it through colour.

Real-world example

Swimming pool test kits add indicator drops to a sample of pool water. The colour is compared with a printed chart so staff can tell whether the water is too acidic or too alkaline, which can irritate swimmers' eyes and damage equipment.

Why?

Why does an indicator change colour at all? Indicator molecules exist in two forms. In acidic solutions one form dominates; in alkaline solutions the molecule gains or loses a hydrogen ion and changes shape slightly. The two forms absorb different colours of light, so we see a different colour.

Common misconception

"An indicator changes the solution into an acid or alkali." It does not. An indicator only reports what is already there. Only a few drops are used, and they have no significant effect on the acidity of the sample.

Worked example

Question: Methyl orange is red in acid and yellow in alkali. A drop is added to an unknown solution, which turns yellow. Is the solution acidic or alkaline?

Reasoning: The yellow colour matches the alkali colour of methyl orange.

Answer: The solution is alkaline (or at least not acidic; methyl orange is also yellow in neutral solution).

Quick check

1. What is an indicator? Answer: A substance that shows different colours in acidic and alkaline solutions.

Exam focus

Learn a clear definition of an indicator and the acid and alkali colours of litmus, phenolphthalein and methyl orange. Be ready to classify examples as natural or synthetic, and explain why universal indicator gives more information than a single indicator.

Advanced insight

Each indicator changes colour over a particular range of acidity, called its transition range . Methyl orange changes in fairly acidic conditions, whereas phenolphthalein changes in mildly alkaline conditions. This is why methyl orange stays yellow in pure water while phenolphthalein stays colourless, and why chemists choose indicators to suit each task.

Summary

An indicator is a substance whose colour depends on whether a solution is acidic, alkaline or neutral. Natural indicators include litmus, turmeric and red cabbage; synthetic indicators include phenolphthalein and methyl orange. Universal indicator is a mixture that shows a range of colours. Indicators report acidity without changing it.

Practice questions

1. Give one natural and one synthetic indicator. Answer: Natural: litmus (or turmeric); synthetic: phenolphthalein (or methyl orange). 2. Phenolphthalein is added to an alkali. What colour is seen? Answer: Pink. 3. Why does universal indicator give more information than litmus? Answer: It is a mixture of indicators that shows many colours, so it indicates how strongly acidic or alkaline a solution is, not just which group. 4. Why is it easier to read an indicator against a white background? Answer: The white background makes the colour clearer and prevents surrounding colours from confusing the result.